Well, perhaps the nebulae are all receding in this
peculiar manner. But the notion is rather startling.
The cautious observer naturally examines other possibilities before
accepting the proposition even as a working hypothesis. He recalls the
alternative formulation
of the law of red-shifts - light loses energy 'in proportion to the
distance it travels through space. The law,
in this form, sounds quite plausible. Internebular
space, we believe, cannot. be entirely empty. There
must be a gravitational field through which the light-quanta travel for
many millions of years before they
reach the observer, and there may be some interaction
between the quanta and the surrounding medium.
The problem invites speculation, and, indeed, has been
carefully examined. But no satisfactory, detailed solution has been
found. The known reactions have been
examined, one after the other - and they have failed to
account for the observations. Light may lose energy
during its journey through space, but if so, we do not
yet know how the loss can be explained.

The observer seems to face a dilemma. The familiar
interpretation of red-shifts leads to rather startling
conclusions. These conclusions can be avoided by an
assumption which sounds plausible but which finds no
place in our present body of knowledge. The situation
can be described as follows. Red-shifts are produced
either in the nebulae, where the light originates, or in
the intervening space through which the light travels.
If the source is in the nebulae, then red-shifts are
probably velocity-shifts and the nebulae are receding.
If the source lies in the intervening space, the explanation of
red-shifts is unknown but the nebulae are sensibly stationary.